1. Field of the Invention
The invention relates to an automatic level control circuit for restricting sound level received from a microphone when the microphone is used in a sound generating device such as a ka-ra-o-ke set. More particularly, it relates to an automatic level control circuit wherein the output level does not saturate even if a sound level received from the microphone exceeds a certain threshold level and therefore the output level changes in response to an input level.
2. Description of the Prior Art
FIG. 7 shows a conventional automatic level control circuit. FIG. 8 shows characteristics of each point (a.about.g) of the conventional automatic level control circuit in FIG. 7. Note that scales of vertical axis in FIG. 8A.about.FIG. 8H are not the same. In the past, if a microphone input exceeded a predetermined threshold level when using a circuit configuration like that of FIG. 7, an input level of amplifier is restricted in order to reduce a distortion. Therefore, an output level of amplifier has been restricted to that purpose. With reference to FIG. 7 and FIG. 8, the operation of the conventional automatic level circuit is explained below.
An input voltage Vi (FIG. 8A) is amplified by a predetermined quantity in the amplifier and outputted to the output terminal. When output Vo of the amplifier becomes equal to or more than a threshold level Vit, distortions of amplifier output increases sharply and the output characteristics become worse. Accordingly the input voltage is divided by a resistor 1 and an N channel transistor Q1 (referred to "transistor Q1" below) so that the output Vo of the amplifier is restricted to a value less than the reference voltage Vref. In this case, the output voltage Vo is represented using an input threshold voltage Vit and gain Gv of the amplifier as follows. EQU Vo=Vref=Gv.multidot.Vit
In other words, when input voltage Vi exceeds the input threshold level Vit, the input voltage Vi is divided by resistor R1 and transistor Q1, and thus voltage Vb applied to the input terminal "b" of amplifier 3 becomes constant (FIG. 8B). For this reason, output voltage Vc of amplifier 3 becomes constant at the point where input voltage Vi exceeds threshold level Vit (FIG. 8C). The output of this amplifier 3 is inputted to voltage comparator 4 to be compared with reference voltage Vref (FIG. 8C). The output of voltage comparator 4 controls a gate voltage Vg (FIG. 8G) of transistor Q1 through a charging/discharging circuit 5. The transistor Q1 controls the input voltage of amplifier 3 by dividing input voltage Vi using resistor 1 and transistor Q1. FIG. 8H shows a resistance value QR1 of transistor Q1. The resistor value RQ is inverse proportional to the gate voltage Vg as described in detail below.
FIG. 9 shows RQ1 versus Vg characteristics of N channel transistor. As shown in FIG. 9, N channel transistor operates such that when gate voltage Vg increases, resistance RQ1 between drain-source decreases. Since this N channel transistor is connected to resistor 1 so as to constitute a series circuit, if gate voltage Vg (FIG. 8G) increases, as understood from FIG. 9, a resistance value RQ1 of the resistor Q1 becomes small, and thereby the input voltage into amplifier 3 becomes small as a result.
FIG. 10 shows a detailed circuit diagram of a charging/discharging circuit 5. In FIG. 7, when plus side input voltage Vd of voltage comparator (COMP) 4 is larger than minus side input voltage Ve, an output voltage Vf of voltage comparator 4 becomes "H" level, which causes switching point in FIG. 10 to connect to power source V.sub.DD, which charges capacitor 6. Therefore, output voltage Vg of charging/discharging circuit 5 becomes higher. Alternatively, when plus side input voltage Vd is smaller than minus side input voltage Ve, the output voltage Vf of voltage comparator 4 becomes "L", which causes switching point in FIG. 10 to connect to constant current source I, which discharges capacitor 6. Therefore, output voltage Vg of charging/discharging circuit 5 becomes low. In this way, charging and discharging of capacitor 6 are repeated for many times, then output voltage Vg of charging/discharging circuit 5 finally makes a curve as shown in FIG. 8G which has a similar shape to that of the input voltage Vi for regions over the threshold level Vit.
Next, an operation of a circuit of FIG. 7 is explained in detail using FIG. 8. In case that output voltage Vo (=Vc) of amplifier 3 is lower than reference voltage Vref, that is, Vo&lt;Vref, the output voltage Vf of voltage comparator 4 becomes 0 [V], therefore, capacitor 6 is discharged in charging/discharging circuit 5 and then gate voltage (FIG. 8G) of N channel transistor Q1 becomes low as described above. Accordingly, ON- resistance RQ1 (FIG. 8H) of transistor Q1 becomes large, which gives a larger voltage to resistance RQ1 of transistor Q1 than resistor 1 in the divider. Thereby, the input voltage to amplifier 3 becomes large. On the other hand, in case of Vo&gt;Vref, output voltage Vg of charging/discharging circuit 5 corresponds to input voltage Vi as described above, and thereby, ON-resistance RQ1 (FIG. 8H) of the transistor Q1 becomes small, which gives a small voltage to resistance RQ1 of transistor Q1 than resistor 1 in the divider. Thereby, the input voltage to amplifier 3 becomes small.
The above-mentioned relationship between output voltage Vo and input voltage Vi of the circuit of FIG. 7 is represented in the following formula (1). EQU Vo=Vc=Vi{RQ1/(R1+RQ1)}.multidot.Gv (1)
where, R1: resistance value of resistor 1, RQ1: ON-resistance value of N channel transistor Q1, Gv: amplification factor of amplifier 3.
In case of Vi.multidot.Gv&lt;Vref, in other words, when input voltage Vi is small, the value of Vo is almost equal to Vi.multidot.Gv. This is due to R1&lt;&lt;RQ1. Still, in case of Vi.multidot.Gv&gt;Vref, in other words, when the input voltage Vi is large, the circuit operates so that the plus side input and the minus side input of voltage comparator 4 becomes ultimately the same value (Vo=Vref). Therefore, output voltage Vo is controlled not to exceed the reference voltage Vref as shown in FIG. 8C. In other words, in the portion where Vi exceeded the threshold Vit, output voltage Vc of amplifier 3 becomes constant.
When a microphone is used for a ka-ra-o-ke set, it is frequently used in a state that a microphone output is at a very large level, that is, in a range of Vi.multidot.Gv&gt;Vref, where Vi is an input voltage, Gv is amplification factor and Vref is reference voltage. In case of Vi.multidot.Gv&gt;Vref, an output of amplifier becomes constant, as described above. In other words, the output voltage of amplifier is saturated as illustrated in FIG. 8C. From this reason, in the range where an input level from a microphone is large, there is a problem that a sense of incongruity resulted from saturation of the output signal occurs. That is, the saturation of the output signal results in that less difference of the output level from the amplifier occurs even if the input level is considerably changed.
It is an object of the present invention to provide an automatic level control circuit which eliminates a sense of incongruity caused from sound volume saturation in the range (Vd=Vi.multidot.Gv&gt;Vref) where an output voltage Vo level of the amplifier is saturated. This is attained by restricting a large input voice level received from microphone but by outputting it in response to the input voice level.